The present invention relates to an electrochemical sensor for detection of a gas in an atmosphere containing that gas, in which the gas must pass through an orifice in the housing in order to be detected by the sensing electrode of the electrochemical sensor, and especially to the embodiments of such electrochemical sensors in which a membrane has been placed over the orifice in order to protect the orifice. In particular, the present invention relates to the provision of channels, especially a plurality of channels, for communication of the gas to or from the orifice so as to prevent blockage of the orifice by a substance or an object.
Electrochemical sensors, including such sensors that are used in the detection of carbon monoxide, normally have a diffusion-limiting gas access hole i.e. an orifice. The gas access hole provides protection for the sensor from excessive exposure to atmospheres which contain high levels of aerosols and other contaminants. It also minimizes changes in volume of the electrolyte when the sensor is subjected to high and low humidity, as significant changes in electrolyte volume potentially lead to leaking of electrolyte as well as changes in sensor sensitivity. The gas access hole also reduces the influx of a target gas into the sensor, so that there will be fresh catalyst in the sensing electrode to ensure a long operational life for the sensor in continued and periodic detection of the target gas, and minimizes any influence of flow of air (atmosphere). The orifice is typically of a diameter of about 0.5-2 mm.
There is always a concern that an orifice of such a small diameter could become partially or completely blocked by an object. The object could be an insect or spider, which often tend to seek a small orifice for protection, for building a nest or out of curiosity. In addition, the orifice could be blocked for other reasons e.g. by condensation of water or by aerosols in the atmosphere which over a period of time could lead to blockage of the orifice.
In order to minimize the possibility of partial or complete blockage of the orifice, and consequent malfunctioning of the sensor, the orifice may be covered by a hydrophobic gas permeable membrane e.g. a membrane formed from Teflon(trademark) fluorocarbon polymer. Examples of the use of a gas permeable membrane include the membrane disclosed in U.S. Pat. No. 4,948,496.
Even with the use of a membrane, the possibility still exists that a substantially gas-opaque deposit could form on the membrane directly above the orifice i.e. the membrane could become coated with a substance that adversely affects transmission through the membrane of the gas that is to be detected. This would retard or prevent diffusion of the gas through the membrane to the orifice and the sensing electrode, and consequently affect performance of the electrochemical gas sensor. Thus, alternate methods of protection of the orifice have been proposed, and one example is the use of a combination of a screen and a porous disc, as disclosed in U.S. Pat. No. 5,284,566, The purpose of the screen is to protect the porous disc from damage. However, such a solution to the problem of protection of the orifice in the electrochemical sensor is relatively expensive in manufacture.
A method of protection of the orifice in a simple and economical manner would be beneficial. It has now been found that gas communication in an electrochemical sensor may be protected in a manner that further reduces the possibility that the orifice will become blocked.
Accordingly, an aspect of the present invention provides an electrochemical sensor for detection of a gas in an atmosphere containing the gas, said sensor comprising a housing having an electrochemical gas sensor with an electrolyte and at least two electrodes, one such electrode being a gas sensing electrode, said housing having an orifice between the sensing electrode and the atmosphere for transmission of gas from the atmosphere to the sensing electrode, said orifice being connected to at least one radial channel extending from said orifice, such that gas communication from the atmosphere through the orifice to the sensing electrode is through said radial channel.
In a further embodiment, the orifice is connected to at least two radial channels, and more preferably to at least three and especially to at least four radial channels, optionally with each of the radial channels connecting with a common channel. The common channel may be an annular channel around the orifice.
In a preferred embodiment of the present invention, the orifice is covered to prevent direct transmission of gas into the orifice.
In a further embodiment, the orifice, radial and common channels are covered by a hydrophobic membrane.
In yet another embodiment, the orifice and optionally the radial channels are covered by a film with low gas transmission characteristics, with gas transmission to the orifice being through said common channel and radial channels.
In another embodiment, the radial channels are disposed symmetrically about the orifice.
In yet another embodiment, the radial channels and common channels are disposed on the membrane side of the orifice, the sensing electrode side of the orifice or on both the membrane and sensing electrode sides of the orifice.